Treatment Matters - Detecting Change In SOM Thermal Fractions with Calorimetry.

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Efforts to characterize soil organic matter (SOM) have been central to the study of soil and site productivity over the last few centuries. Over this time various approaches have been developed to characterize chemical qualities and detect change based on ecosystem, management and disturbance. These efforts support our central understanding of SOM as a dynamic body but, overall, lack the resolution and robustness for quantification in a rapid reproducible manor. Calorimetry has an analogous place in chemistry and science by characterizing a broad range of organic and inorganic materials and their mixtures illustrating composition, purity and stability. Differential Scanning Calorimetry (DSC) is an advanced application that provides precise quantification of material mass and energetic qualities, under controlled atmospheric and heating conditions, ideally suited to characterize SOM status and complexity.

Our objective was to use DSC to quantify the effect of fertilizer and herbicide application on SOM thermal fractions (energy quantified across temperature regions - 200-350, 350-500 and 500-650 C) in forest soils under Ponderosa Pine plantations. Soils were sampled from two sites of the Garden of Eden silviculture experiment in California. Both sites have a common experimental design (replicated complete randomized block) and taxonomic classification (Halpohumults) but vary in site productivity. Samples were taken from three depth increments (0-10, 10-20 and 20-30 cm) and processed for analysis.

Fertilizer application increased energetic storage across all thermal fractions and depths at both sites versus control. Fertilization ended > a decade prior to sampling - treatment legacy? Herbicide application reduced energetic storage across all thermal fraction at both sites for surface soils and generally reduced measurement variability. With reduced understory presence, how important are litter inputs of varying qualities for sustaining SOM pools and characteristics? Overall, DSC illustrated that treatment matters and its value as a tool to investigate SOM.